It is well known in the art of structural composite materials to employ balsawood as the material of a core of a sandwich panel comprising outer layers of a fibre reinforced matrix resin composite material. The sandwich panel is typically manufactured by disposing respective fibre layers on opposite surfaces of the balsawood and then infusing a curable resin into the fibre layers and against the opposite surfaces during a vacuum assisted resin transfer moulding step. The resin is then cured to form the sandwich panel.
There is a need to provide a strong adhesive bond between the cured resin layers and the core, so that there is a high peel strength between the cured resin and the core.
There is also a need to minimise the resin take-up of the core. This adds undesired weight to the sandwich panel. The opposite surfaces of the balsawood core tend to have a propensity to take-up the curable resin by absorption of the resin into the opposite surfaces, when the resin is infused against the surfaces during a vacuum assisted resin transfer moulding step.
Balsawood is a natural material and so has a structure and properties which are not particularly uniform. In order to provide a core having high shear strength, the balsawood is cut into sheets, with the major planar cut surfaces of the sheets being substantially transverse to the height direction of the balsawood tree. Therefore the cut surfaces expose the ends of vessels, typically 0.2 to 0.4 mm in diameter, which are acicular cells which form the major part of the Balsa tree water transport system. In a cut sheet for manufacturing a core, the vessel portions extend between the major planar cut surfaces of the sheet. Axial parenchyma cells, typically 0.02 to 0.04 mm in diameter, and fibres also extend between the major planar cut surfaces of the sheet. Radial parenchyma cells extend substantially in or parallel to the major planar cut surfaces of the sheet.
The outer core surfaces are required to form a strong bond, exhibiting high peel strength, with the resin of the fibre reinforced matrix resin layer laminated to the core. The provision of the transverse cut surfaces of the balsawood exposing the vessels and axial parenchyma cells can assist the formation of a high peel strength bond between the resin matrix and the core.
However, such transverse surfaces, by exposing the ends of the vessels and the ends of the axial parenchyma cells, tend to absorb a large amount of resin which is infused into the fibrous reinforcement material during the vacuum assisted resin transfer moulding step. The absorbed resin in the core adds significant weight to the sandwich panel, without increasing the mechanical properties of the sandwich panel, which is undesirable. Also, the absorption of resin into the balsawood core increases raw material costs during manufacturing.
It is known to coat the balsawood surfaces with a thick polymer layer, prior to the vacuum assisted resin transfer moulding step. Usually a liquid thermoset resin such as epoxy resin is used with a thixotropic additive to thicken the mix. This is manually screeded over the balsawood surface and allowed to cure to pre-treat the surface prior to resin infusion. The thixotropic agent helps to prevent the tendency for the liquid resin to fill the cells while waiting for the resin to thicken and cure. The thixotropic mixture has relatively high viscosity and results in a thick and heavy coating, usually in excess of 400 g/m2 on each face of the balsawood. There is a combined weight saving compared to infusing the balsawood without any coating as the coating blocks the majority of the cells after cure. However, this process adds substantial time and cost to the manufacturing process and still results in a high resin absorption.
It was also known to apply solvent based coatings to coat the surface to leave a thinner barrier coating. However, this type of layer has been found significantly to reduce the peel strength of the resin layer on the balsawood core surfaces. The combined result is that this known coating does not provide the combination of high mechanical properties, including high peel strength, and low resin uptake required by balsawood cores for use in sandwich panels.
Furthermore, when applying polymeric resin coatings to balsawood the curing and/or drying time of the coatings can be rather long, for example to enable solvents to evaporate fully from the coating, which makes the manufacturing process inefficient.
In combination, there is a need for sandwich panels incorporating a balsawood core to exhibit a combination of high mechanical properties, including high peel strength, and low resin uptake, and which is efficient, easy and inexpensive to manufacture.